Trudeau has criticized Trump in the past so it's just a revenge quest
Trump wants to be emperor and trying to rule over neighbors is just what emperors do
Just some delusionial Trump bullshit that demonstrates his mental decline
(highly speculative) Influence from Elon, whose grandfather wanted to turn Canada into an NRx-style white supremacist corprate-fascist state 80 years ago and failed, hence the move to south africa. Conceivably Elon wants Trump to make him governor of Canada so he can finish the job. Generally I'm not in favor of wild intruguing speculation like this but it's just too fun/horrifying to pass up.
Russia has long threatened Canada from the north, but Canada's alliance with the USA deterred it from attacking. Now the USA is considering stealing Canada's resources and gaining access to strategic Arctic waters, Russia will also know that Canada is vulnerable because the USA will no longer defend it. So now these two predators are circling Canada and may even be coordinating plans with one another.
I extremely doubt that Trump would give any of Canada to Russia. Just because he wants Canada doesn't mean he wouldn't interfere with a Russian invasion, to the contrary. He would probably use it as a pretext to anex Canada, but either way that's never gonna happen. Russia doesn't have the strength to invade Canada and it would be immensely stupid to do so.